1. Field of the Invention
The present invention relates to a silicon wafer cleaning fluid that is useful, for example, in the process of producing semiconductor silicon wafers and in the process of producing semiconductor devices and to a silicon wafer cleaning method using said cleaning fluid.
2. Description of the Prior Art
As cleaning fluids that are widely used in removing intense contamination with heavy metal contaminants on the surface of semiconductor silicon wafers, for example, a fluid which consists of hydrochloric acid, hydrogen peroxide, and water and is called SC-2 (hereinafter referred to as SC-2) used in the RCA method, and oxidizing cleaning fluids, such as nitric acid and aqua regia, are known. The cleaning of silicon wafers using these cleaning fluids is generally carried out such that silicon wafers are supported by fluororesin carriers and are dipped in these cleaning fluids for a prescribed period. When silicon wafers are dipped in the above cleaning fluids, heavy metals on the silicon wafer surface are dissolved and removed. However, with respect to the effect of removing heavy metals by these cleaning fluids, the examination of the contamination with gold that is most hardly removed among heavy metals reveals that it is recognized that aqua regia and SC-2 are effective in cleaning intense contamination with gold amounting approximately to 10.sup.15 atoms/cm.sup.2 or more, but the cleaning effect is not satisfactory if the contamination is not so high as this level and cleaning effect can not be obtained if the contamination is on the order of 10.sup.13 atoms/cm.sup.2 or below. This is because if the contamination is 10.sup.14 atoms/cm.sup.2 or less, at the time of cleaning, a spontaneous oxide film is formed on the wafer surface and gold atoms are taken under it.
Therefore, it is desirable that while the spontaneous oxide film is removed such an oxidizing agent is worked. The present inventor proposed in Japanese Patent No. 613521 a cleaning fluid comprising 1 volume of aqua regia and 0.3 volume or less of HF as an agent for removing the spontaneous oxide film. According to the method described in the above patent, it is shown that contamination with gold amounting to 10.sup.15 atoms/cm.sup.2 can be reduced by one cleaning treatment to 10.sup.10 atoms/cm.sup.2. However, the etching action on the silicon wafer surface caused by the HNO.sub.3 and HF is strong because it is facilitated with chlorine and nitrosyl chloride generated from the aqua regia, at a certain concentration of HF this cleaning fluid etches the silicon wafer to an amount of 1 .mu.m or more in some cases, and the fine indentations (haze) generated on the silicon wafer damages the specular state of the surface. If the cleaning is effected with the HF concentration of the cleaning fluid lowered extremely to an extent where haze will not be generated, because a small difference in the state of the generation of nitrosyl chloride and chlorine causes the etching amount to scatter considerably, the conditions of the cleaning becomes hard to control. Thus, this cleaning fluid is not suitable for the process of producing IC, LSI circuits, etc.
Therefore, where particularly intense contamination with heavy metals is required to be removed, a treatment of dissolving heavy metals with SC-2 or aqua regia and an etching treatment of removing the spontaneous oxide film with dilute hydrofluoric acid are alternately repeated. However, although this cleaning method can remove metals that can be easily oxidized, such as copper, the method cannot exhibit satisfactory cleaning effect on contamination with gold and, for example, even if the dilute hydrofluoric acid/SC-2 treatment is repeated 10 times, contamination with gold is reduced only to about 1/2.
On the other hand, in the process of producing semiconductor devices, there is a step of giving strong, though shallow, defects to the silicon wafer surface, such as reactive ion etching (RIE) and ion implantation. Since generally heavy metals are high in diffusion speed, they penetrate easily into such a defect region and therefore in that step there is a fear that intense contamination with heavy metals will occur. Accordingly, for such contamination, a cleaning fluid is required that can etch and remove the defect region itself. On the other hand, the etching should not damage the smoothness of the silicon wafer surface. Thus, a cleaning fluid and cleaning method that can control the etching to a thickness of several tens .ANG., and particularly about 20 to 30 .ANG., are required. This is because if the etching is to this extent, the smoothness is not damaged. Further to attain high productivity, desirably it is demanded that the cleaning can be carried out by simple contact treatment for a short period of about 1 min.